Hydrocarbons may be accompanied by water in underground rock formations. Such water typically may have a high concentration of dissolved salt. After disturbance of a formation by hydrocarbon production operations, the water in the formation may have a variable concentration of dissolved salt. The variation may be over time and location within the formation. This variation may result from the mixing of various water containing fluids used during production with each other and with the water in the formation.
Knowledge of the salt concentration, typically referred to as salinity, in the water of a formation is useful in the process for estimating the volume of hydrocarbons in the formation and thereby in managing production. A salinity measurement is needed because some instruments used for performing downhole measurements in a formation are sensitive to salinity. For example, an instrument may not be able to provide accurate estimates of the water and hydrocarbon fractions without knowledge of salinity. An example of an instrument sensitive to salinity is a borehole resistivity sonde, which is used to measure electrical resistivity of fluid in the pores of a rock. From the resistivity measurement, the fraction of water within pores may be determined. However, since resistivity is sensitive to salinity, knowledge of the water salinity is necessary to compute an accurate estimate of the water fraction in the pores. Since the salinity may vary over time and depth within a formation during various stages of hydrocarbon production, an accurate downhole measurement of salinity is needed over time and as a function of depth within a formation in order to optimize hydrocarbon production.